case-control study

— A type of non-randomized study comparing the characteristics of people with a particular health condition (cases) with the characteristics of people without that condition (controls), to find what may have caused the problem

Case-control studies are also sometimes undertaken to evaluate effects that are subsequently confirmed by other studies.

Example:

For example, a comparison of people admitted to hospital with heart attacks (cases) with others admitted with different diagnoses found that the people with heart attacks were less likely to have used aspirin. The apparent protective effect of aspirin against heart attack was subsequently confirmed in randomized studies.